This invention relates in general to apparatus for detecting the velocity of blood flowing in a vessel of the eye, and more particularly, to a fundus camera-based retinal laser Doppler velocimeter. The apparatus provides for absolute measurement of the speed of red blood cells flowing in individual retinal vessels, and can be routinely used by medically trained persons, without the need for placement of a contact lens on the patient's eye or resort to invasive techniques.
As was pointed out in U.S. Pat. No. 4,142,796, issued Mar. 6, 1979, to Charles E. Riva, and elsewhere, the ability to measure the velocity of blood flowing in a single blood vessel or in a capillary bed is very useful for medical purposes. Impairment of the blood flow in the tissues of the ocular fundus, or the retina of the eye, is associated with a large number of diseases that can lead to grave visual disorders. Blood flow measurements have also been used to differentiate between living and nonliving tissues, to determine which tissues require surgical removal and which should not be removed.
The feasibility of using laser doppler velocimetry (hereafter referred to as "LDV") to measure blood flow in individual retinal vessels was demonstrated in 1972 by Riva et al (C. E. Riva, B. Ross and G. B. Benedek, Investigative Ophthalmology, Vol. 11, pps. 936 et seq., Nov., 1972), who measured the Doppler-shift frequency spectrum of laser light scattered from red blood cells flowing in a retinal artery of an anesthetized rabbit. The maximum Doppler frequency shift f.sub.max arising from the light scattered by the red blood cells flowing at the maximum speed V.sub.max was estimated from the spectrum. V.sub.max was calculated from f.sub.max and from estimates of the intraocular scattering geometry using the general relation: ##EQU1## where .lambda. is the wavelength in vacuo of the incident laser light, n is the refractive index of the flowing medium, .theta..sub.i, is the intraocular angle between the incident beam and the flow direction, and .theta..sub.s is the intraocular angle between the collected scattered light and the flow direction. It was assumed that the incident laser beam was perpendicular to the flow direction.
It has been shown (G. T. Feke and C. E. Riva, J. Opt. Soc. Am. Vol. 68, pps. 526 et seq., 1978) that f.sub.max can be determined from Doppler-shift frequency spectra obtained from human retinal vessels using short measurement times (less than one second), and further, it has been shown that V.sub.max can be determined by a procedure involving the collecting of light scattered by red blood cells in two distinct directions separated by a known angle (C. E. Riva, G. T. Feke, B. Eberli and V. Benary, Applied Optics, Vol. 18, pps. 2301 et seq July 1, 1979). Analysis of the collected light, it has been found, yields an absolute measure of V.sub.max that is independent of the exact orientation of the vessel and of the relative angular orientation of the incident and scattered light beams with respect to the flow direction. In the present apparatus, bidirectional LDV is performed using apparatus, the basic component of which is a standard retinal camera. The need for a contact lens is eliminated, and the laser beam is delivered to the eye through the fundus illumination optical system of the camera. Use of the present apparatus markedly simplifies the technique of retinal blood flow measurement.
Thus, bidirectional laser Doppler velocimetry allows absolute measurements of the speed of red blood cells in retinal vessels. In this technique, Doppler shift frequency spectra (hereafter referred to as "DSFS") of laser light scattered from the red blood cells are recorded for two directions of the scattered light, while the direction of the incident beam remains constant. These DSFS, when obtained in short measurement times, exhibit large fluctuations in spectral power up to a cutoff at a frequency f.sub.max that arises from light scattered by red blood cells flowing at the maximum speed, V.sub.max, at the center of the vessel. .DELTA.f=f.sub.2 max -f.sub.1 max is the difference between the cutoff frequencies obtained from DSFS recorded in two directions, K.sub.1 and K.sub.2. V.sub.max is obtained from these DSFS using the relation: ##EQU2## .lambda. is the wavelength of the incident laser beam, n, the index of refraction of the flowing medium, .alpha., the angle between K.sub.1 and K.sub.2 and .beta. is the angle between the vector V.sub.max and its projection on the plane defined by the vectors K.sub.1 and K.sub.2.
The first absolute measurements of V.sub.max were obtained using a standard slitlamp microscope in conjunction with a low-vacuum corneal contact lens. Use of a contact lens simplifies considerably the determination of the scattering geometry because the lens eliminates the corneal refraction of the Doppler shifted light. Several problems arise, however, when the technique is applied to human subjects: (a) there is a risk of corneal abrasion or infection; (b) there is poor stabilization of the target retina to motion because the fellow (non-target) eye is used for target fixation; (c) changes in intraocular pressure caused by application of the contact lens may affect retinal blood flow; and (d) the slitlamp instrument in its present stage of development does not allow DSFS to be simultaneously recorded for two directions of the scattered light or the determination of V.sub.max in vertical vessels.
U.S. Pat. No. 4,166,695, issued Sept. 4, 1979, to Hill et.al. discloses use of a retinal (fundus) camera in the measurement of retinal blood flow, but the Hill et. al. apparatus does not provide absolute measurements of the speed of red blood cells in retinal vessels.
This invention relates, therefore, to a new bidirectional laser Doppler instrument that uses a fundus camera instead of a slitlamp, thus avoiding the need for a contact lens, but still allowing straightforward determination of the scattering geometry, the scattered light being detected in two directions. Moreover, as will be seen, the instrument allows V.sub.max to be determined for blood vessels extending in any direction, and provides target fixation for the eye under examination.